TLS (Transport Layer Security) encrypts the traffic between server and client.
If you’re familiar with SSL, you get the idea.
To use this feature, your IMAP server must offer TLS
capability, plus PHP 4.3.x with OpenSSL support.
Note that the ‘STARTTLS’ command is not supported; the server must
have a dedicated port listening for TLS connections.If your IMAP server is localhost, you can safely disable this.
If it is remote, you may wish to seriously consider enabling this.
Enable TLS (y/n) [n]: n

Each IMAP server has its own quirks. As much as we tried to stick
to standards, it doesn’t help much if the IMAP server doesn’t follow
the same principles. We have made some work-arounds for some of
these servers. If you would like to use them, please select your
IMAP server. If you do not wish to use these work-arounds, you can
set this to “other”, and none will be used.
bincimap = Binc IMAP server
courier = Courier IMAP server
cyrus = Cyrus IMAP server
dovecot = Dovecot Secure IMAP server
exchange = Microsoft Exchange IMAP server
hmailserver = hMailServer
macosx = Mac OS X Mailserver
mercury32 = Mercury/32
uw = University of Washington’s IMAP server
other = Not one of the above servers
[courier]: courier